How do you build a progress bar with a fill button in JavaScript?
Use setInterval to increment progress: const interval = setInterval(() => { progress += 1; bar.style.width = progress + '%'; if (progress >= 100) clearInterval(interval); }, 50). Use CSS transition for smooth animation.
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More FAQs in Progress Bar Interview Questions
Use XMLHttpRequest (not fetch). Listen for xhr.upload 'progress' event: if (e.lengthComputable) { const percent = (e.loaded / e.total) * 100; setProgress(percent); }. fetch does not support upload progress events.
Add role='progressbar', aria-valuenow (current value), aria-valuemin='0', aria-valuemax='100'. Use aria-live='polite' to announce changes for screen readers. This makes the progress bar usable with assistive technology.
Use a striped or pulsing CSS animation without a specific percentage. The animation shows that something is happening, but the total is unknown. This is common for loading states where the server response time is unpredictable.
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